The demand for thermoplastic resin products to which electric conductivity is imparted by incorporating carbon black into thermoplastic resins has recently been considerably increased in various fields, in particular, materials for electronic parts, computers and VTR, antistatic materials for household appliances and materials for electromagnetic shielding.
As methods for preparing such conductive thermoplastic resins, there have conventionally been used those in which a continuous twin-screw extruder is used. As methods for feeding ingredients to the extruder in the production method which makes use of such a continuous twin-screw extruder, there have in general been used a method in which carbon black and a resin are, in advance, blended in, for instance, a Henschel mixer or a tumbling mixer and then the resulting blend is fed to the extruder by a variety of feeders; and a method in which carbon black and a resin are separately and quantitatively fed to the extruder using counting feeders. If adopting either of these feeding methods, however, resins and carbon black differ from one another in true specific gravity, bulk density and shape and accordingly, they cause classification during blending or after being fed to the extruder to thus cause a difference in the concentration of carbon black in the composition. Therefore, the resulting resin cannot sometimes be practically used since these methods do not provide resins which do not have any variation in the conductivity and have stable conductivity.
The bulk density of carbon black used for imparting conductivity to resins is very low and accordingly the carbon black includes a large amount of air. Such a large amount of air is released when the carbon black is dispersed in resins using a twin-screw extruder and rises from the feed opening for the carbon black. The rise of the air leads to scattering of carbon black. As a result, there have been pointed out a variety of problems such that the scattering of carbon black leads to a difference in the carbon black concentration within the resin composition, that the scattering of carbon black also impairs the working environment, and that the suspended carbon black fine particles are adhered to the inner portions of precision machines to thus destroy the internal electronic parts thereof. The blowing up of carbon black is closely correlated to the feed rate thereof and accordingly, the feed rate should be reduced if carbon black causes scattering and this in turn results in a decrease of the productivity.
As a production method capable of being adopted when ingredients are fine particles, there has been proposed a production method in which an opening is positioned downstream of a feed opening for ingredients and the structure of the screw at the portion between the ingredient-feeding part and the opening is designed in such a manner that it does not compress the ingredients, but melts or knead the same (Japanese Un-Examined Patent Publication (hereinafter referred to as "J.P. KOKAI") No. Sho 58-29644). However, this method does not permit the improvement of the blend of a resin and carbon black in case of the conductive carbon black used in the present invention.
Alternatively, there has also been proposed a method which makes use of a resin master batch having a high carbon black content for the purpose of, in particular, preventing any scattering of carbon black (J.P. KOKAI No. Sho 54-58747) and there has in general been adopted a method for imparting conductivity to a resin by kneading a desired resin with pellets of such a resin master batch having a high carbon black content. In this connection, a small amount of the master batch pellets is in general mixed with a thermoplastic resin and then the resulting mixture is kneaded and formed into conductive thermoplastic resin articles.
In such a method, however, it is relatively easy to impart, to a resin, stable conductivity within a low resistance region (the surface resistivity thereof falling within the range of from 10.sup.2 to 10.sup.4 .OMEGA.), while it is quite difficult to control the conductivity of the resulting resin composition in a high resistance region (the surface resistivity thereof falling within the range of from 10.sup.6 to 10.sup.16 .OMEGA.).
On the other hand, if the content of conductive carbon black present in a conductive master batch pellets is very close to that of conductive carbon black to be present in a desired conductive thermoplastic resin product, pellets having a conductive carbon black content identical to that in the intended conductive thermoplastic resin product have been produced, in one step, by the same method used for preparing the master batch in place of preparing the resin product by blending master batch pellets separately prepared with a small amount of thermoplastic resin and then conductive thermoplastic resin products have been directly prepared from the resulting pellets. In this method, however, it is likewise relatively easy to impart, to a resin, stable conductivity within a low resistance region (the surface resistivity thereof falling within the range of from 10.sup.2 to 10.sup.4 .OMEGA.), while it is quite difficult to control the conductivity of the resulting resin composition in a high resistance region (the surface resistivity thereof falling within the range of from 10.sup.6 to 10.sup.16 .OMEGA.).